Many head mounted displays offer a fully immersive visual environment, which may be termed virtual reality display, i.e. the user does not observe any outside scene and instead only observes images provided by the display. Providing displays in helmets, e.g. head-up displays in aviation helmets, is also known. These displays typically provide a display imaged at infinity superimposed onto the outside world image. Conventional examples of these displays have performance limitations that arise from a trade-off between providing as wide a field of view as possible with the increase in the weight of the projection system required for increasing width of field of view. In relation to certain helmets, e.g. helmets for soldiers, it has been proposed to provide transparent displays on visors of the helmets. Generally, transparent displays may be considered as displays which have some degree of transparency, i.e. that are not fully opaque. One recently developed example of transparent display is transparent organic light emitting diodes (OLED) type display, which may also be referred to as active matrix transparent organic light emitting diodes (AM-TOLED) type display.
Quite separate from use of transparent displays, pupil tracking is known in the use of helmets that provide a fully immersive visual environment. Pupil tracking apparatus tracks the positions of the user's pupils and the information of the pupils' positions is used accordingly.
Quite separate from the fields of displays in helmets and transparent displays, switchable microlens arrays are known. See for example “Microlenses immersed in nematic liquid crystal with electrically controllable focal length” L. G. Commander, S. E. Day, C. H. Chia, and D. R. Selviah, in Third European Optical Society ‘Microlens Arrays’ Topical Meeting, 1995, US20070183020 and U.S. Pat. No. 7,408,601B1.